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In this Jan. 19, 1959 file photo, Pfc. Warner Bitterman, left, watches as Army chief chemical officer Maj. Gen. Marshall Stubbs, center, checks new civilian gas mask being worn secretary Margaret Francis at his Pentagon office in Washington. For some baby boomers, North Korea's nuclear advances and the Trump administration's bellicose response have prompted flashbacks to a time when they were young, and when they prayed each night that they might awaken the next morning. For their children, the North Korean crisis was a taste of what the Cold War was like.

After the Soviet Union collapsed in 1991, the era of nuclear nightmares — of the atomic arms race, of backyard bomb shelters, of schoolchildren diving under desks to practice their survival skills in the event of an attack — seemed to finally, thankfully, fade into history.

For some baby boomers, North Korea's nuclear advances and President Donald Trump's bellicose response — though he did compliment Kim Jong Un in a tweet on Wednesday — have prompted flashbacks to a time when they were young, and when they prayed each night that they might awaken the next morning. For their children, the North Korean crisis was a taste of what the Cold War was like.

"I'm not concerned to where I can't sleep at night. But it certainly raises alarms for Guam or even Hawaii, where it might be a real threat," said 24-year-old banker Christian Zwicky of San Bernardino, California.

He isn't old enough to remember the popular 1950s public service announcement in which a cartoon character named Bert the Turtle teaches kids how to dive under their desks for safety. But Zwicky did see it often enough in high school history classes that he can hum the catchy tune that plays at the beginning. That's when Bert avoids disaster by ducking into his shell, then goes on to explain to schoolchildren what they should do.

"I do remember that," says 65-year-old retiree Scott Paul of Los Angeles. "And also the drop drills that we had in elementary school, which was a pretty regular thing then."

Even as a 10-year-old, Paul said, he wondered how much good ducking under a desk could do if a bomb powerful enough to destroy a city fell nearby. No good at all, his teacher acknowledged.

Then there were backyard bomb shelters, which briefly became the rage during the missile crisis of 1962, when it was learned the Soviets had slipped nuclear-tipped missiles into Cuba and pointed them at the United States.

After a tense, two-week standoff between President John F. Kennedy and Soviet leader Nikita Khrushchev that some believe brought the world the closest it's ever come to nuclear war, the missiles were removed and the shelters faded from public interest.

"When Trump took office it doubled our sales, and then when he started making crazy statements we got a lot more orders," says Walton McCarthy of Norad Shelter Systems LLC of Garland, Texas. "Between now and a year ago, we've quadrupled our sales."

His competitor, California-based Atlas Survival Shelters, says it sold 30 shelters in three days last week. During its first year in business in 2011 it sold only 10.

Nathan Guerrero, a 22-year-old political science major from Fullerton, California, agrees, saying he learned in history class that the "shining example" of a way to resolve such a conflict was how Kennedy's brother and attorney general, Robert Kennedy, brokered the tense negotiations.

"But knowing the way the current administration has sort of been carrying itself, it doesn't look like they are keen to solving things diplomatically," he said.

"I'd be lying if I said such crazy things haven't crossed my mind," he said, laughing nervously. "But in reality it doesn't strike me as I'd be ready to go shopping for bunkers yet." Instead, he studies for law school and tries "not to think too much about it."

Other Americans are more sanguine about the possibility of nuclear war. Rob Stapleton has lived in Anchorage, Alaska, since 1975, and he is aware that Alaska has been considered a possible target because it is within reach of North Korean missiles.

"There's been some discussion about it around the beer barrel and I'm sure the United States is taking it seriously, but we're not too concerned around here," he said.

Alaska is so vast and spread out, said Stapleton, that he and his friends can't imagine why North Korea would waste its time attacking The Last Frontier.

"I mean sure you'd be making a statement, but you'd not really be doing any damage."